This invention relates to display devices, especially ones that use an organic material for light emission.
One type of electroluminescent display device is described in PCT/WO9013148, the contents of which are incorporated herein by reference. The basic structure of this device is a light-emitting polymer film (for instance a film of a poly(-phenylenevinylene)xe2x80x94xe2x80x9cPPVxe2x80x9d) sandwiched between two electrodes, one of which injects electrons and the other of which injects holes. The electrons and holes excite the polymer film, emitting photons. These devices have potential as flat panel displays.
Another type of organic light-emitting device is a small molecule device, details of which are given in U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference. These have a light-emitting layer which comprises at least one small molecule material such as tris(8-hydroxyquinoline)aluminium (xe2x80x9cAlq3xe2x80x9d) sandwiched between the two electrodes.
In an organic light-emitting device the organic light-emitting layer is generally divided into individual pixels, which can be switched between emitting and non-emitting states by altering the current flow through them. The pixels are generally arranged in orthogonal rows and columns. Two alternative arrangements for controlling the pixels are generally used: passive matrix and active matrix. In a passive matrix device one of the electrodes is patterned in rows and the other in columns. Each pixel can be caused to emit light by applying an appropriate voltage between the row and column electrodes at whose intersection it lies. This calls for high peak brightnesses from the pixels, because each pixel can only be powered for a fraction of the scan cycle. In an active matrix display the high peak brightness is not required because each pixel can be left in an emitting state whilst another pixel is addressed.
FIG. 1 illustrates a circuit for driving one pixel in a thin-film transistor (xe2x80x9cTFTxe2x80x9d) active matrix display. The circuit comprises the pixel itself, illustrated as diode 1, which is connected between electrodes 2 and 3. Electrodes 2 and 3 are connected to all the pixels of the device and a voltage sufficient for emission from the pixel is applied constantly between the electrodes 2 and 3. At least part of the switch circuit 4, which in practice is embodied by thin-film transistors, lies between electrode 3 and the pixel 1. (There may also, or alternatively, be circuitry between the pixel/diode 1 and the electrode 2). The switch circuit is controlled by way of row and column electrodes 5,6. To cause the pixel 1 to emit light, voltages are applied to the electrode 6, to switch the switching transistor 7 on, and to electrode 5 to charge the storage capacitor 8. Electrodes 6 is then turned off. Since the capacitor 8 is charged the current transistor 9 is switched on and the voltage applied at electrode 3 is applied to the pixel, causing it to emit. Although it requires a more complex circuit than a passive matrix device this arrangement has the advantage that the pixel can be held in an emitting state by means of the capacitor 8 whilst other pixels on different rows and columns are addressed by their row and column electrodes.
FIG. 2 shows a schematic plan view of typical switching circuitry associated with a pixel of an organic light-emitting device and FIG. 3 shows a cross-section of the circuitry of FIG. 2 on the line 1A-1Axe2x80x2. The circuitry comprises a scan (or gate) line 10 (which corresponds to the electrode 6 in FIG. 1), a signal (or data) line 11 (which corresponds to the electrode 5 in FIG. 1), a common line 12 (which corresponds to the electrode 3 in FIG. 1), a switching thin film transistor shown generally at 13 (which corresponds to the transistor 7 in FIG. 1), a storage capacitor shown generally at 14 (which corresponds to the capacitor 8 in FIG. 1) and a current transistor shown generally at 15 (which corresponds to the transistor 9 in FIG. 1). As FIG. 3 shows, insulating layers 16 of SiO2 separate the component parts of the circuitry, and the circuitry is deposited on a glass substrate 17. At the output of transistor 15 is a contact region 29 which constitutes the output terminal of the circuit.
Banks 30 of insulating material (not shown in FIG. 2) are formed to constrain the edges of the light-emitting region itself. To connect between the output terminal of the TFT circuit and the light-emitting material of the pixel there is an electrode 19 of transparent indium-tin oxide (xe2x80x9cITOxe2x80x9d). This makes contact with the contact region 29 and provides a wide pad which forms the anode of the emitting device. A layer 33 of light-emitting material is deposited on the pad (corresponding to pixel 1 in FIG. 1) and finally a cathode 31 (corresponding to electrode 2 in FIG. 1) is deposited on top of it. Light emission from the pixel towards a viewer is generally in the direction into the page in FIG. 2 and as shown by arrow B in FIG. 3. Therefore, to prevent it obscuring the emitted light, the TFT circuitry is located generally to the side of the light-emitting material 33.
ITO has good transparency, low sheet resistance and established processing routes, and it has a low resistance which makes it especially useful in passive matrix displays where, because each pixel can only emit for part of the time, high peak through-currents are needed. However, the processing of the ITO can cause problems. Typically, the ITO is deposited as a continuous layer (e.g. by sputtering or evaporation) over the entire device. It must then be pattemed to give separate pads 19 for each pixel of the device. The patterning is typically lithographic, with the ITO being etched to remove the unwanted areas. This causes problems because the materials used for the etching can easily seep into the TFT structure, through voids between the component regions, and damage the circuitry. Damage to the circuitry of only one pixel of a display may cause the entire display to be rejected.
Devices have been made more stable by using a layer of a conductive polymer between the ITO and the light emitting layer (see, for example, J Carter et al., Appl. Phys. Lett. 71 (1997) 34), and in other fields, such as the field of non-emissive devices the ITO layer has been omitted (see, for example, A Lien et al., xe2x80x9cConducting Polyaniline as a Potential ITO Replacement for Flat Panel Applicationsxe2x80x9d, Proceedings of the International Display Research Conference, Society of Information Display, Toronto, Sep. 15-19, 1997, p. 1).
According to the present invention there is provided a method for forming a display device, comprising: depositing a thin-film transistor switch circuit on a substrate; depositing by ink-jet printing an electrode layer of light transmissive conductive organic material in electrical contact with the output of the thin-film transistor circuit; and depositing an active region of the device.
Preferably the active region is also deposited by ink-jet printing.
The active region may preferably be in the form of a layer.
The active region may be a light-emitting region (for instance comprising an organic light-emitting material) or a region the passage of light through which can be controlled (for instance comprising a liquid crystal material).
The said organic light emitting material may be a polymer material. The organic light-emitting material is preferably a conjugated material. A suitable material is a semiconductive conjugated polymer such as PPV, or a derivative thereof. The light-emitting material suitably is or comprises PPV, poly(2-methoxy-5(2xe2x80x2-ethyl)hexyloxyphenylene-vinylene) (xe2x80x9cMEH-PPVxe2x80x9d), a PPV-derivative (e.g. a di-alkoxy derivative), a polyfluorene and/or a co-polymer incorporating polyfluorene segments, PPVs and/or related co-polymers. As an alternative to ink-jet printing it could be deposited by spin-coating, dip-coating, blade-coating, meniscus-coating, self-assembly etc. The constituent of the light-emitting region and/or its precursor may be water-based: examples are precursor-based PPVs. An alternative material is an organic molecular light-emitting material, e.g. Alq3, or an organic oligomer light-emitting material, or any other small sublimed molecule or conjugated polymer electroluminescent material as known in the prior art.
The electrode layer suitably has a sheet resistance less than 200 kOhm/square and preferably in the range from 20 to 200 kOhm/square. The thickness of the electrode layer is preferably in the range from 20 to 100 nm.
The electrode layer is preferably deposited on to an insulating layer. The insulating layer suitably forms a cover for the thin-film transistor circuit.
The electrode layer is suitably transparent or semi-transparent (e.g. with a transparency of greater than 50%), at least at the frequency of any light emissions from the active layer, and suitably in the visible spectrum.
The conductive organic material of the electrode layer is suitably a conductive polymer material. The conductive organic material may be a polyaniline (e.g. a doped polyaniline or polyaniline derivativexe2x80x94see M. Angelopoulos et al., xe2x80x9cApplications of Conducting Polyanilines in Computer Manufacturing Processesxe2x80x9d, Intrinsically Conducting Polymers: An Emerging Technology, M. Aldassi (ed.), Proceedings of the NATO Advanced Research Workshop on Applications of Intrinsically Conducting Polymers, Burlington Vt. USA, pp 147-156, 1993 Kluwer Academic Publishers, Netherlands), a polythiophene (e.g. a doped polythiophene or polythiophene derivative), a polypyrrole, a doped conjugated polymer such as doped PPV. One preferred material is polystyrene sulphonic acid doped polyethylene dioxythiophene (xe2x80x9cPEDT/PSSxe2x80x9d).
The electrode layer suitably comprises a lower electrode layer in electrical contact with the output of the thin-film transistor circuit and an upper electrode layer adjacent to the active region. The upper electrode layer is preferably of a different composition and/or has different electrical properties from the lower electrode layer. For example, the lower electrode layer preferably has a higher electrical conductivity (suitably with a sheet resistance below 1000 Ohm/square) than the upper electrode layer (suitable with a sheet resistance above 200 kOhm/square). Where the electrode comprises PEDT/PSS the upper electrode layer has a higher concentration of polystyrene sulphonic acid (xe2x80x9cPSSxe2x80x9dthan the lower electrode layer.
The output of the thin-film transistor circuit suitably comprises a contact for making electrical connection to the electrode. The contact suitably extends in the major. plane of the electrode for improving electrical contact between the two and/or charge distribution in the electrode. For example, the contact suitably extends along one or more sides of the electrode layer. The contact may surround or substantially surround the electrode layer, at least in the plane of the layer. The contact is preferably of a material of a higher conductivity than that of the electrode layer. To avoid direct carrier injection from the contact into the active region (if the device is configured so that the two are adjacent) the work function of the material of the contact is preferably lower than that of the material of the electrode layer and/or the contact may be coated with an insulating layer over its surface adjacent to the active region. The work function of the material of the contact is suitably less than 4.5 eV and preferably less than 4.3 eV. The material of the contact is preferably inert to acids, such as sulphonic acid. Preferred materials for the contact include metals, such as aluminium and aluminium alloys, and conductive refractory materials such as titanium nitride. Preferably the conductivity of the contact is greater than that of the material of the electrode.
The electrode layer preferably abuts the said insulating layer over one major surface. The other major surface of the electrode layer preferably abuts a major surface of the active region. The other major surface of the active region preferably abuts another electrode layer. Either electrode layer may be the anode or the cathode.
Where the active layer can control the polarisation of light passing through it the device may also preferably comprise one or more polarising layers and/or a reflective layer and/or a light-emitting layer, as is well known in the fabrication of prior liquid crystal devices.
According to a second aspect of the invention there is provided a method for forming an organic light-emitting display device, comprising: depositing a thin-film transistor switch circuit on a substrate; depositing an electrode layer of light transmissive conductive organic material in electrical contact with the output of the thin-film transistor circuit; and depositing an organic light-emitting layer over the electrode layer. The details of this aspect of the invention are as for the first aspect of the invention.
The methods described above may suitably allow for the formation of a device having a plurality of pixels as described above, preferably with a common substrate but individual thin-film transistor switch circuits and electrode layers and separate organic light-emitting regions for each pixel. The electrode layer (or a sub-layer of the electrode layer) may extend contiguously across all the pixels of the device, particularly where that layer or sub-layer has a sheet resistance greater than 200 kOhm/square. The device may be a multi-colour display, having a set of pixels of different emission colours, e.g. red green and blue.